1. Field of the Invention
The present invention relates to storage-stable thermosetting compositions comprising bis-imides of unsaturated carboxylic acids.
2. Description of the Prior Art
The copolymerization of bis-imides of unsaturated carboxylic acids with comonomers which can be polymerized by free-radical or ionic initiation, and yielding heat-stable polymers upon curing under the influence of heat, is known to this art. As a general rule, these polymers are prepared by homogenizing the various constituents by melting/intimately admixing same at moderate temperatures, in a first stage, and then, in a second stage, and after shaping, by curing the resulting composition or "prepolymer" to provide a polymer which is infusible and insoluble in organic solvents. Most frequently, the composition in prepolymer stage is stored for subsequent use. Depending upon the initial composition thereof, the extent of the reaction and the type of application envisaged, the prepolymer, at ambient temperatures, is either a more or less viscous resin or is a powder having a more or less elevated softening point. Since the reaction is most typically carried out in the presence of an excess of bis-imide, relative to the stoichiometry of the addition reaction, it frequently results that a portion of the bis-imide which has not yet taken part in the polymerization reaction crystallizes in the admixture upon cooling, thus giving rise to a heterogeneous composition. In particular in the case of resins which are fluid at moderate temperatures, this phenomenon represents a major disadvantage to the user, who must then heat the resin until the bis-imide melts (generally at above 100.degree. C.), in order to be able to use the resin, and this prevents their use at moderate temperature. Furthermore, certain applications require resins which are fluid at relatively low temperature.